In the current year we have conducted several studies investigating the regional vascular changes that occur in young and aged hypertensive rats which alter their ability to maintain cerebral blood flow (CBF) during a hypotensive stimulus. In early work we have validated the use of radioactive microspheres for repeated measures of regional blood flow (Life Sciences, in press). Publication of this work was essential in conducting the experiments as originally proposed. Using this work as a base, we have completed studies testing the ability of young and aged hypertensive rats to maintain CBF during hypotensive stimuli (Brain Research, in press). We observed that young normotensive rats are able to maintain CBF at control levels during hypotension induced by ganglionic blockade. Maintenance of CBF during hypotension was inhibited in hypertensive rats and was significantly worse in aged hypertensive rats. These results indicate that cerebrovascular changes take place as a function of hypertension and aging which inhibit the ability of the cerebrovascular system to respond to hypotensive stimuli. In other experiments completed but not published, we have tested the ability of hypotensive drugs used clinically to maintain CBF. It was found that sodium nitroprusside produced responses similar to ganglionic blockade while nitroglycerin was a more effective agent for maintaining CBF in hypertensive rats. One problem encountered in these experiments using hypotensive drugs was tachyphylaxis. We found a high incidence of tachyphylaxis in hypertensive rats. This complicated the work and required many more rats than expected. In future work, we intend to use a combination of ganglionic blockers and hypotensive drugs to obviate this problem.